1. Field of the Invention
The present invention relates to an autofocusing system, and more particularly to an autofocusing system for use in, for example, an electronic image pickup apparatus which obtains an electrical image signal through the use of an image pickup device which performs; a photoelectric conversion of an image of a subject formed through an imaging optical system.
2. Description of the Related Art
In recent years, electronic image pickup apparatus have come into widespread use. An example of an electronic image apparatus is an electronic still camera which performs; a photoelectric conversion of an image of a subject. The image is received through an imaging optical system and transformed into an electric signal through the use of an image pickup device. The apparatus further records an image signal thus obtained on a recording medium.
Such electronic image pickup apparatus, include a means to detect a focal position (the position of a focal point), and is equipped with an autofocusing system (which will hereinafter be referred to simply as an AF means). The autofocusing system automatically accomplishes the focal adjustment on the basis of a focal position obtained by the focal position detecting means.
Among conventional autofcusing systems applicable to electronic image pickup apparatus, for example, there have been known a so-called contrast detection AF means (which will be referred hereinafter to as an imager AF. In this technique, the detection of a focal position of a subject image is made on the basis of a difference (contrast) of high-frequency components contained in an image signal received through an image pickup device. The imager AF means technique further uses an infrared active type AF means (which will be referred to hereinafter as an infrared active AF means) which is a trigonometric survey technique where an illuminating light beam, such as a infrared ray, is applied to a subject. A reflected light beam, occurring due to the reflection of the illuminating light beam on the subject, is received and used for calculating the distance to the subject based on an angle made between the illuminating light beam and the reflected light beam.
This infrared active AF means performs open loop control in which a group of predetermined lenses (which will be referred to hereinafter as a focus lens group), of an imaging optical system, are shifted by a predetermined preset distance in accordance with a focal position detection result thereby focusing the device.
In an electronic image pickup apparatus (or the like) using the infrared active AF means, the time is short needed between the start of a focal position detecting operation (generation of a release instruction signal) until the actual completion of the autofocusing operation (AF operation). This an advantage is that the time is short between the generation of a release signal to the start of the actual exposure operation, the release time lag. Additionally, even if the environment is in a low luminance condition for photography, the focal position detection result is surely obtainable.
On the other hand, there is a limit to the distance that the illuminating light beam can reach. In the case of a subject existing remotely, there difficulty in attaining a precise focal position detection result. Further, when the focus lens group is shifted in accordance with a focal position detection result by the infrared active AF means, external factors become relevant. For example, if a lens barrel holding an image pickup optical system to cause a mechanical positional difference is used, difficulty can be experienced in shifting the focus lens group to an appropriate position.
The imager AF means also employ feedback control (the so-called mountain-climbing type AF means). In these devices a high-pass filter (HPF) is used to extract a high-frequency component from an image signal obtained by an image pickup device. The focus lens group of the imaging optical system is moved in a direction of its optical axis, and the extracted high-frequency component is compared in quantity with each high-frequency component corresponding to each focus lens position. The highest contrast point (point assuming a peak value) is thus detected and is indicative of the largest high-frequency component quantity.
Thus, in an electronic image pickup apparatus using the imager AF means, if the detection of the contrast in an image taken is possible, the focal position detection result is surely obtainable regardless of the distance from a subject. On the other hand, since the high-frequency component quantity is detected at each position while the focus lens group is shifted, the time needed for the focal position detecting operation becomes long. This causes a disadvantage in that the release time lag is prolonged. Additionally, for example, if the environment is in a low luminance condition at photography, or if a desired subject is in a low contrast condition, there is a possibility that the contrast of an image taken cannot be surely detected depending on the condition of the subject. Such a situation can prolong the time needed for the focal position detecting operation and can make it difficult to detect the focal position.
For this reason, a conventional electronic image pickup apparatus is equipped with an infrared active AF means and an imager AF means, and selectively used the two AF means to obtain an appropriate focal position detection result when needed such a device is shown in, for example, Japanese Unexamined Patent Publication 5-119250 and others.
The autofocusing system disclosed in Japanese Unexamined Patent Publication 5-119250 is designed such that the focusing operation is conducted by the imager AF means during ordinary imaging operation. Only in imaging environments where difficulty is encountered in detecting a focal position through the use of this imager AF means, the device switches to the infrared active AF means to conduct the focal position detecting operation and the AF operation with respect to a desired subject. Thus, this can provide a proper focal position detection result regardless of the degree of brightness of the subject.
In the infrared active AF means, when a neighboring subject is photographed, the optical axis of the light emitting/receiving means, (the light beam path) does not coincide with the optical axis of the imaging optical system. This causes a small difference, or parallax, between the focal position detecting area in a picture determined by the AF means and the position of the desired subject in the picture taken. Generally this parallax increases as the distance between the electronic image pickup apparatus and the subject becomes shorter.
When photographing a subject where the distance to the subject is small (near photography or proximity photography), the depth of field decreases. As a consequence even if a focal position detecting operation is conducted for a focal position detecting position set at a position shifted from the subject position due to the occurrence of the parallax, the focal position detection result can contain errors. As a result, the subject may be out of focus in the picture.
Incidentally, the means disclosed in Japanese Unexamined Patent Publication 5-119250 and others are made such that the two AF means are selectively chosen based only on the brightness of a subject. These devices do not solve problems including the occurrence of the parallax in near photography at all. These devices do not disclose an approach to eliminate various problems arising due to the occurrence of the parallax.
The present invention was developed in consideration of the above-mentioned problems.
Accordingly, it is an object of the present invention to provide an autofocusing system, for use in an electronic image pickup apparatus, which is capable of surely conducting a focal position detecting operation at a high speed to obtain a highly accurate focal position detection result even in near photography.
Briefly, in accordance with an aspect of this invention, an autofocusing system is characterized by comprising an image pickup device for conducting a photoelectric conversion of an image of a subject, formed through an imaging optical system, to produce an electric image signal, image pickup means including the imaging optical system for picking up the subject image, driving means for driving the image pickup means in a first preset range or in a second range smaller than the first range, high-frequency component detecting means for detecting a high-frequency component in an image signal corresponding to a subject image formed while at least a portion of the image pickup means is shifted by a predetermined quantity through the use of the driving means, focusing drive means for adjusting a focal condition of an image, produced by the image pickup means, on the basis of an output of the high-frequency component detecting means, infrared light detecting means including light-emitting means for radiating infrared light toward the subject and light-receiving means for receiving reflected light produced by reflection of the infrared light from the light-emitting means on the subject and made to detect an output signal corresponding to a distance to the subject on the basis of an output of the light-receiving means, calculating means for calculating a distance to the subject on the basis of an output of the infrared light detecting means, and control means for driving the image pickup means in said first range when the distance to the subject calculated by the calculating means is shorter than a predetermined distance and for driving the image pickup means in the second range when the distance to the subject calculated by the calculating means is longer than the predetermined distance.
This object, and another objects and advantages, of the present invention will become more readily apparent from the following detailed description.
According to this invention, in an electronic image pickup apparatus equipped with an imager AF means and an infrared active AF means, there is offered an autofocusing system capable of surely conducting a focal position detecting operation at a high speed even in a near photography irrespective of a distance to a subject to obtain a high-accuracy focal position detection result at all times.